Subminiature circuit protectors are useful in applications in which size and space limitations are important, for example, on circuit boards for electronic equipment. Cartridge type circuit protectors, basically comprising fuse elements in glass sleeves, are known to be reliable, particularly when the fuse element is hermetically sealed in the glass sleeve. Making hermetically sealed subminiature glass sleeve circuit protectors in the reduced size required for computer circuit boards, however, is labor intensive and relatively expensive. This typically involves mechanically attaching lead wires or connectors to the fuse element, and using a heat cured epoxy resin to form the hermetic seal. In addition, these manufacturing difficulties impose limitations on how small the circuit protectors can be made.
The present invention, generally, provides a simple and relatively inexpensive method of manufacturing a subminiature cartridge type circuit protector.
The present invention also provides a subminiature circuit protector made by the method of the invention that has an improved operating lifetime and improved interrupting capability.
More particularly, the present invention provides a method for making a subminiature glass cartridge circuit protector having a substrate carrying a metal film fuse element connected to leads, the metal film fuse element and portions of the lead elements being hermetically sealed in a glass sleeve.
According to another aspect of the invention, a gas is trapped in the glass cartridge to provide a non-oxidizing environment for improving the operating lifetime of the fuse element. A gas with arc quenching properties may be selected to improve the current interrupting capability of the circuit protector.
Alternatively, the environment in the glass sleeve may be air, or air at a pressure less than atmospheric pressure.
According to the method of the present invention, a substrate having a metal film fuse element is placed in a glass sleeve, and leads and solder preforms are placed in contact with the contacts of the fuse element. The assembly is placed in an environmentally controllable chamber, which is at least partially evacuated. The chamber then may be charged with a selected gas. The assembly is heated to a temperature sufficient to soften the glass and melt the solder, and the pressure in the chamber is increased so that the ends of the glass sleeve deform about the leads and form a hermetic seal. Heating causes the solder to melt and form a connection between the leads and the contacts of the fuse element substrate.
The environment in the assembly may be air. The pressure in the chamber may be decreased to a pressure below atmospheric pressure to provide a partial vacuum environment. Alternatively, the pressure may be increased to substantially atmospheric pressure.
According to another embodiment of the invention, a selected gas is introduced into the chamber after evacuation. In one embodiment, the selected gas is an inert gas, such as nitrogen. When the seals form, the gas is captured in the sleeve, and provides an environment that prolongs the operating lifetime of the fuse element.
According to an alternative embodiment, the selected gas is sulfur hexafluoride. Sulfur hexafluoride enhances the interrupting capability of the fuse element.